We have developed databases and software useful for comparative analysis of protein three-dimensional structure. These tools are distributed freely to biologists and developers of biotechnology software. MMDB (Molecular Modeling DataBase) is the 3D-structure component of the Entrez molecular biology retrieval system. MMDB is an ASN.1 database where all data items describing macromolecular structure are validated and explicitly listed, so that application software need not contain the complex logic required to retrieve this information from text formats such as PDB files. Work this year has concentrated on addition of accurate taxonomy assignments for macromolecular structures within MMDB, creation of new message and data types for transmission of structure-structure alignment data to local viewers, and on construction of an automated monthly update and indexing system, Pubstruct. CN3D (see in three dimensions) is a multi-structure visualization program distributed as part to the Entrez client software and in a stand-alone version lauchable via the MIME protocol in World- Wide-Web Entrez. The software differs from other public domain viewers in supporting display of multiple aligned structures from Entrezs structure neighbor database, and in supporting simultaneous highlighting/picking of multiple sequence and multiple structure alignments. Other features added this year are on-the-fly alignment of the sequences of homologs, so that an Entrez user may easily map conserved sequence features onto the know 3D structure. These software features are intended to facilitate molecular biologists identification of important structure-function relationships within protein families. - macromolecular 3D structure database molecular modeling - Neither Human Subjects nor Human Tissues